Low power lasers are used to read and write data on the data side of optical media, such as compact discs (CDs), digital versatile discs (DVDs), and the like. Typically, a laser beam writes various types of data on a data side of a disc while the disc is rotating. Data may be recorded by making marks on the disc, representing binary digits. Various data writing strategies have been employed to create desirably shaped marks that are precisely positioned and that have sharp edges to enable detection.
Labels on such optical discs are typically used to provide descriptive human-readable indicia of the data content recorded on the disc, as well as illustrations, artwork, and the like. Such descriptions and images are generally handwritten, affixed, or marked on the side of the disc opposite the data side. Recently, apparatus and methods have been developed with the ability to generate an image or label on non-data areas, such as the non-data side (the “label side”) of an optical disc, using a low power laser, such as the same laser that is employed to read and write digital or electronic data on the data side of the disc. For example, see U.S. Patent Application Publication No. 2003/0108708, Anderson, et al. which is commonly assigned with the present application.
Conventional optical disc drives have a focusing servo incorporating a focus actuator, generally including a voice coil motor. The focus actuator moves an objective lens in a Z-axis direction relative to the disc, to generally maintain the lens at a constant optimal position, sometimes called “best focus” position, relative to the disc. The “best focus” position is typically optimized for reading and writing data on the data side of the disc, such as by selecting a default position where the laser beam is focused on a spot at a layer of recording material below the surface of the data side. The use of a defocused spot for writing disc labels is described by Anderson, et al., in U.S. patent application Ser. No. 10/732,047, filed Dec. 9, 2003, which is commonly assigned with the present application. By defocusing a laser spot rather than using a focused laser spot, a larger marking spot can be achieved on optically labeled disc media without a sacrifice in speed.
In order to accurately position the objective lens at a desired focal position, such as an offset from the “best focus” position, it is desirable to know the amount of movement of the focus actuator given an applied voltage or current. In particular, it is helpful to know or learn the electrical to mechanical gain of the voice coil motor of the focus actuator. This gain may be used to determine the variations in voltage that must be applied to the voice coil motor to allow the positioning of the objective lens at a desired offset from best focus. A value for the gain is generally specified in a data sheet provided by a manufacturer of the voice coil motor, but such figures are typically of limited value, due to variances that may in some cases be plus or minus forty percent (±40%) for a focus actuator voice coil motor installed in an optical drive. Such a wide variation may result in laser-produced markings that undesirably have degraded image quality.